The present invention relates to a connection structure for metallic members, which is, for example, suitable as a connection structure between a tank and a pipe of a fuel tank for automobiles, etc., and in particular, relates to a technique in which corrosion resistance and sealing can be improved without using Pb.
For example, in a fuel tank 1 for automobiles, as shown in FIG. 10, pipes such as a filler neck pipe 2 for refueling, a breather pipe 3 for breathing air in refueling, and a venting pipe 4 for releasing pressure in the fuel tank 1, are connected. In the case in which such pipes are connected to the fuel tank 1, one end of a pipe P is press-fitted in the fuel tank 1, as shown in FIG. 11, then ring shaped solder S is adjacently placed at a boundary between the pipe P and the fuel tank 1, and this solder S is heated and melted by an electrode 6 for high frequency induction heating. Then, the melted solder S solidifies at the corner portion of the boundary between the pipe P and the fuel tank 1, as shown in FIG. 12, and both members are thereby airtightly connected.
As a conventional solder, Pbxe2x80x94Sn alloy has generally been employed. However, it is not desirable to use Pb since there are environmental regulations on Pb leached from industrial waste such as shredder dust, etc., and substitutes for Pb have been required. Therefore, Ag alloys, Cu or Cuxe2x80x94Zn based alloys, Znxe2x80x94Al alloys, etc., have recently been used, and in Japanese Unexamined Patent Publication No. 71488/98, a Sn alloy solder (Snxe2x80x94Ag based) has also been disclosed.
On the other hand, as a material for fuel tanks and pipes, surface treated steel sheets which are subjected to Zn plating, Al alloy plating, Zn alloy plating, etc., may be used. Alternatively, after-treatment plating which is carried out after material is processed from a steel plate, may be carried out. In any case, a fuel tank 1 and a pipe P on which are formed platings M1 and M2, are connected together by solder S, as shown in FIG. 12. The solder S is applied to such connections since the heating temperature thereof is lower than those of other methods such as welding, etc., the heat distortion at a position where dimensional accuracy is required can be suppressed in a thin layer, the sealing thereof is superior, equipment can be miniaturized, etc.
In a fuel tank for automobiles, seals which can withstand high internal pressure are required at soldered portions since internal vapors of fuel expand with increasing temperature, and in addition, reliability and durability in which functions thereof are not damaged by vibration and acceleration during driving of the automobile, are also required. The fuel tank is often installed under the floor of the car body, and therefore, a high level of coating and high corrosion resistance are required, even at the soldered portions, since they are exposed to severe road environments and climatic conditions, such as snowmelt salt, mud, water, humidity, are splashed gravel. Furthermore, corrosion resistance of the inner surface is also required, since corrosive components such as acids and peroxides are produced when gasoline decomposes in a fuel tank.
However, there was a problem in that surrounding plating is heated by heating during soldering, and the plating is thereby heat-deteriorated. That is, it is necessary to heat to a temperature 50xc2x0 C. or more (preferably 100xc2x0 C.) higher than the melting point of solder, in order to securely fix members by increasing the solderability (wettability). In particular, a pipe wall portion just above a high frequency heating electrode is heated to a high temperature by this heating and plating metal is alloyed with Fe material, and the corrosion resistance is thereby lowered and the plating is weakened thereby. Depending on conditions, a plating M2 may melt and flow down, as shown in FIG. 13(a), or a porous oxide film M3 may be formed by oxidizing the plating M2, as show in FIG. 13(b), and corrosion resistance is thereby drastically lowered, and the application property is also lowered in the latter case.
In addition, in the case in which the solder material is different from the plating material, there was a problem in that the corrosion resistance is lowered by contact corrosion occurring in which the base metal acts as an anode. Therefore, it is an object of the present invention to provide a connection structure for metallic members and a connecting method in which high reliability and high durability can be ensured even in use under severe conditions. Specifically, in the present invention, plating and solder materials are selected in consideration of following points.
{circumflex over (1)} A solder having superior solderability to plating is selected in order to obtain a connection structure having superior qualities in which a soldered portion thereof has high strength and there are few internal defects.
{circumflex over (2)} Materials for the solder and the plating having small differences in corrosion potential are selected in order to reduce contact corrosion between the solder and the plating.
{circumflex over (3)} A plating having high corrosion resistance to saltwater or decomposed gasoline is selected in order to improve the corrosion resistance of the plating.
{circumflex over (4)} A solder having a low melting point is selected in order to reduce heat deterioration of the plating caused by heating during soldering and to increase thereby adhesion of films. In particular, it is desirable that melting points of the solder and the plating be 180xc2x0 C. or more in view of the fact that baking finishing is carried out at 150xc2x0 C. or more in a subsequent process.
(1) Prevention of Heat Deterioration of Plating
The present inventors researched materials for plating and solder from the above points of view. Firstly, melting points of various alloys or metals were enumerated in the following, in order to take into account the heat deterioration prevention of the plating. In comparison with properties shown in Table 1, Zn to Cuxe2x80x94Zn based alloy (lines 6 to 10) have high melting points, and it is therefore anticipated that heat deterioration, such as oxidation, etc., of the plating material (Pbxe2x80x94Sn alloy, etc.) will occur.
(2) Corrosion Resistance of Plating
In a fuel tank for automobiles, corrosion resistance to the external environment and corrosion resistance to acids and peroxides produced by the degradation of fuel are required. Then, with respect to the internal corrosion resistance which prevents Fe in a saltwater environment from causing corrosion and the external corrosion resistance which has resistance to gasoline degradation products including formic acid or acetic acid, the evaluations of various metals are described in Table 2. As is apparent from Table 2, Alxe2x80x94Si alloy and Snxe2x80x94Zn alloy are preferable as a plating material.
(3) Contact Corrosion Resistance and Solderability
The order of corrosion potential of various metals in a saltwater is shown in FIG. 1. In the case in which two metals shown in FIG. 1 are employed for soldering and plating, if the locations of the corrosion potential of the two metals are far apart, the difference in the corrosion potentials is high, and the base metals are easily corroded. According to this rule as a standard, the contact corrosion resistances of the combinations of various metals were evaluated and the results are shown in Table 3. Additionally, with respect to each combination of metals, the solderabilities thereof were also evaluated. In this evaluation, the following criteria were used: cases where the solderability thereof was the same as that of combination of Pbxe2x80x94Sn alloy plating and Pbxe2x80x94Sn alloy solder are indicated by ∘, cases where it was slightly inferior to the above, but was at an allowable level are indicated by xcex94, and cases where soldering thereof was difficult or impossible are indicated by xc3x97, and these results are shown in Table 3.
As is apparent from Table 3, the combination of Snxe2x80x94Zn alloy plating and solder was best in the contact corrosion resistance. However, this combination has inferior solderability and would be difficult to use in practice because zinc oxide is a product on the surface of melted metal in soldering. In contrast, the combination of Snxe2x80x94Ag alloy solder and Snxe2x80x94Zn alloy plating has superior solderability, and the contact corrosion resistance is also at an allowable level. In addition, these alloys also have low melting points and superior internal and external corrosion resistances.
Therefore, the present invention has been made based on the above tests and is characterized in that in a connection structure for metallic members in which a first metallic member and a second metallic member are connected by solder, Snxe2x80x94Zn alloy is plated on at least one of the first and second metallic members, and the solder consists of Snxe2x80x94Ag alloy.
In the connection structure for the metallic member as composed above, solder and plating easily melt each other since the melting points thereof are close, and thus, the solderability is superior, the number of internal defects is small, and the metallic members are firmly adhered. A seal which resists high internal pressure in the interior of a fuel tank and which has reliability and durability and are not damaged by vibration and acceleration in the driving of automobiles can thereby be obtained. In contrast, in the case in which the difference between the melting points of solder.and plating is large, it is necessary to conform the heating temperature to a higher melting point. Therefore, the metal having a lower melting point is oxidized by heating and oxide film is formed, and Fe, which is base, is thereby easily corroded and the adhesion of the films is decreased. However, such problems do not occur in the present invention. Additionally, the internal corrosion resistance and the external corrosion resistance are superior since the plating consists of Snxe2x80x94Zn alloy, and generation of contact corrosion is also reduced since differences in corrosion potentials are small.
Here, a Zn-rich layer in which Snxe2x80x94Zn alloy plating is alloyed with the solder is desirably provided on the surface.of a portion of which the solder and the plating melt with each other. By providing the Zn-rich layer, contact corrosion between the plating and the solder is prevented, and a chemical conversion coating is easily formed in pre-treatment processing and the adhesion of films is thereby improved.
In addition, the Snxe2x80x94Zn alloy plating desirably has a composition of 93 to 55% by weight of Sn and 7 to 45% by weight of Zn. When the content of Zn is below 7% by weight, the amount of Zn to prevent corrosion of Fe is low, and Fe is thereby easily corroded, and the corrosion resistance in a saltwater environment is decreased. In contrast, when the content of Zn exceeds45% by weight, Zn oxide in a porous state is formed on the surface of a portion of which the solder and the plating melt with each other. Therefore, the solderability deteriorates and the fastening strength is decreased.
Furthermore, a connecting method for metallic members, according to the present invention, is characterized in that a connecting method for metallic members in which a first metallic member and a second metallic member are connected by solder, comprises plating Snxe2x80x94Zn alloy on at least one of the first and second metallic members, using Snxe2x80x94Ag alloy as a solder, and connecting the metallic members while cooling at the connected portion. According to the present invention, formation of porous layers by washing away the plating due to overheating or by oxidation thereof can be reliably prevented. In particular, when one metallic member is a hollow member such as a pipe, a moderate suitable cooling effect can be obtained preferably by supplying cooling gas such as air or other gas into the inside of the hollow member. In addition, the soldering conditions have been controlled by electric power supplied to an electrode for high frequency heating heretofore. However, electric power control has recently been further expanded by further adding cooling thereto, and the control has become easy and the quality thereof has also been stabilized.